The human heart is made up of four chambers: the left and right atria and the left and right ventricles. Simply put, the right atrium receives blood from the veins of the body and the right ventricle pumps this blood through the pulmonary arteries to the lungs. The left atrium receives blood from the lungs through the pulmonary veins and the left ventricle pumps this blood through the aorta systemically to the tissues of the body.
The left ventricle (LV) is commonly analyzed by physicians in order to determine the presence of coronary circulatory problems in a patient. Coronary artery disease can be diagnosed through observation of the functioning of the left ventricle. Roughly eighty percent of all heart disease is coronary artery disease.
While as many as three hundred different parameters regarding heart functioning are known to exist, the most common and widely used parameter for evaluating the LV function is the ejection fraction. The ejection fraction (EF) is the percentage of blood ejected or displaced from the LV with each contraction. A fully contracted ventricle is in what is known as the systolic state, while a fully dilated ventricle is in the diastolic state. Therefore, the EF equals the difference in blood volume in the LV between the systolic and diastolic states divided by the blood volume in the LV in the diastolic state. Thus, ##EQU1## where EF is expressed as a percentage
V.sub.D =blood volume in LV in diastolic state PA1 V.sub.S =blood volume in LV in systolic state.
An EF in the range of fifty to sixty percent can be expected from a normal, healthy heart. An EF in the range of thirty to forty percent is a sign of improper functioning, while an EF of twenty percent is accompanied by serious consequences.
LV functioning has been observed by ultrasound detection of the heart wall, known as the endocardium. However, the observation of the dilation and contraction of the LV is an indirect method of measuring the ejection fraction, since the EF is a percentage of blood ejected. This indirect method is inaccurate because of poorly defined endocardiums, ventricular hypertrophy and misshapen cardiac chambers, i.e. aneurysms. Therefore, to accurately analyze LV functioning the volume of blood must be measured directly.
The field of nuclear imaging or nuclear medicine has been used to image a blood pool in the LV. Nuclear imaging is an invasive diagnostic technique which requires an injection of a radioactive isotope into the blood stream. This radioactive isotope is normally tagged or attached to a pharmaceutical drug before injection into the patient's blood stream. The isotope radiates nuclear energy which can be detected by a sensor, commonly known as a gamma camera, pointed at the area of interest, in this case the LV. When averaged over up to one thousand heart cycles or over twenty minutes, this nuclear imaging technique can produce an image of the blood pool in the LV both in the systolic and diastolic states. These images can either be manually analyzed to determine the EF and other LV functional parameters, or the images can be automatically analyzed by an image processor.
To analyze a "black-and-white" image with an image processor, the image is divided into an array of rows and columns of hundreds of picture elements, or pixels. Each pixel is assigned a value representing the "shade of gray" in that pixel. These values can then be analyzed by the image processing computer, with the use of known pattern recognition algorithms, to determine the boundaries of the blood pool in the LV. Once the boundaries are determined, known estimation algorithms are utilized to estimate the three-dimensional volume of the blood pool from the two-dimensional image of the blood pool. The EF and other LV functional parameters can then be calculated.
There are several drawbacks to nuclear imaging to determine LV function. The most serious is the invasive nature of injecting a radioactive isotope tagged to a pharmaceutical drug into the blood stream. The technique is also inaccurate due to the need to average the image over a twenty minute period. During this period the patient may move or the heart cycle or rate may speed up or slow down. Either of these two changes will cause inaccuracies in the image.
A second invasive diagnostic technique involves the use of a catheter to inject a dye into the bloodstream near the heart. This dye is selected to be absorbent to x-ray energy. Standard x-ray techniques are then used to image the blood pool in the LV by transmitting x-ray energy through the LV from one side of the body while detecting x-ray energy on the other side of the body. The dye in the LV absorbs the x-ray energy and creates a shadow in the image which can thereafter be evaluated. Again, this invasive method has the disadvantage of injecting a foreign substance into the blood stream. Furthermore, x-ray techniques work best for still images and not dynamic or moving images.
In addition to the use of ultrasound to observe a moving wall of the heart, ultrasound has been used to observe or measure the flow of blood. By definition, ultrasound is a sound pressure wave having a frequency greater than twenty kilohertz. Most ultrasound sensors utilize the Doppler effect to sense motion. In simple terms, the Doppler effect is the frequency shift resulting from the reflection of a constant frequency signal off of a moving object. An object moving toward the signal will reflect a higher frequency signal. Conversely, an object moving away from the signal will reflect a lower frequency signal. The magnitude of the frequency shift is proportional to the speed of the moving object. Stationary objects will not change the frequency of the reflected signal.
In the case of monitoring blood flow with ultrasound, the moving objects are the red blood cells in the bloodstream. An ultrasound transducer, for converting an electrical signal to transmitted ultrasound and for converting received ultrasound to an electrical signal, is placed over the area of interest in a patient's body. This area of interest for observation of LV functioning with ultrasound is the bottom or apex of the heart. A two-dimensional image is generated by sweeping the transmitted direction of the ultrasound through a fixed angle, resulting in a wedge or sector shaped image.
It is common to use color video to represent the image received by the ultrasound sensor. The movement of blood toward the transducer (a positive Doppler frequency shift) has commonly been represented by red, an arbitrarily selected but universally applied color. The movement of blood away from the transducer (a negative Doppler frequency shift) has commonly been represented by blue, another arbitrarily selected color. Slow moving or stationary objects, including blood, are represented by grey. Greater rates of movement are represented by saturated shades (more white), and slower rates of movement are represented by less saturated shades (less white). Hence, such color ultrasound systems are known as color flow imaging systems.
One of the important aspects of color flow imaging involves sampling theory. Sampling theory reveals that the accuracy of the Doppler frequency shift determination is proportional to the time spent observing or measuring the movement of the object creating the shift. Since, as mentioned previously, the magnitude of the frequency shift is proportional to the speed of the moving object, slow moving objects will have small frequency shifts. Therefore, relatively more samples will be needed to accurately determine the speed of slow and stationary objects. The dynamics of the heart are such that the endocardium of the heart moves at a velocity which is believed to be in the range of ten centimeters per second (with some variation depending on exercise level and physical condition), while blood flow is at a velocity in the range of thirty to one hundred twenty centimeters per second. Flow rates near ninety centimeters per second typify leaks from valves and holes in vessels and chambers of the heart.
The color flow imaging systems can be optimized to look for velocities of a particular magnitude. When a moving object with a velocity outside of the limits of a selected range is encountered, there will exist an uncertainty as to the measurements of this out-of-limit velocity due to sampling theory limitations. When this occurs the movement is portrayed by a green color mixed with either red or blue, as appropriate. The green color was arbitrarily selected, and although not universal has become somewhat of a standard in the industry. Increasing degrees of uncertainty cause increasing amounts of the red or blue colors to be replaced with green. Adding the green to red obtains shades of yellow, and adding the green to blue obtains shades of cyan.
Since color flow imaging systems have primarily been used to detect leaks and defects or to measure the speed of blood flow, the velocity range is normally set to a maximum setting for observation of LV functioning, e.g. from thirty to one hundred twenty centimeters per second. This setting is then adjusted in an effort to reduce the amount of green in the object of interest. Such common usage has, however, not resulted in a significant capability to evaluate LV function including EF and other heart functional characteristics involving the movement of blood pools.
It is with respect to these and other considerations that the present invention has evolved.